1. Field of the Invention
The present invention relates to a grid in a fuel assembly for supporting a plurality of fuel rods and, more particularly, to an improvement in spring elements which are urged against the fuel rods to support the same.
2. Prior Art
A fuel assembly, which is mounted to a nuclear reactor, is formed such that a plurality of elongated fuel rods each having sealed thereto a plurality of cylindrical fuel pellets are arranged in spaced parallel relation to each other through a predetermined spacing. A plurality of grids are used as means for arranging the fuel rods in the above positional relationship.
As shown in FIGS. 5 through 7 of the attached drawings, the aforesaid grid comprises a plurality of elongated straps 1 each in the form of a thin plate. A plurality of slits 2 formed in the straps 1 in equidistantly spaced relation to each other longitudinally along each of the straps 1 are fitted into each other such that the straps 1 intersect with each other in the form of a lattice to define therein a number of grid cells 3 each having a peripheral wall formed by wall sections of the respective four straps 1.
Pairs of dimples 5 and a spring element 6 for supporting a corresponding one of a plurality of fuel rods 4 are formed on the peripheral wall of a corresponding one of the grid cells 3 in facing relation to each other. The fuel rod 4 is urged against the dimples 5 by the spring elements 6 so that the fuel rod 4 is supported therebetween. Each of the spring elements 6 is formed as follows. That is, a pair of portions of the peripheral wall of the grid cell 3, which are located respectively at both lateral sides of an elongated portion of the strap 1 serving as the spring element 6, are punched to form a pair of punched sections 7 and 7. The elongated portion of the strap 1, which remains at the center between the pair of punched sections 7 and 7, bulges in a stepwise manner. The elongated bulging portion of the strap 1 has a planar section 6a which is in pressure contact with the fuel rod 4.
The spring element 6 constructed as above has its spring characteristics, that is, a deflection-load curve as shown in FIG. 8. In the spring element 6, within a range indicated by A, that is, within a range in which, even if the amount of deflection varies, the load is constant, the spring element 6 urges the fuel rod 4 against the dimples 5. Accordingly, in the grid formed with such spring elements 6, even if slight variation occurs in positions of the respective spring elements 6 during assembly of the grid, the variation is absorbed to clamp the fuel rod 4 with constant load.
In the grid constructed as above, however, when the fuel rod 4 is inserted into the grid cell 3, an outer peripheral surface of the fuel rod 4 slides against the spring elements 6 and the dimples 5. Accordingly, there is reason for concern that scratches will be generated axially on the outer peripheral surface of the fuel rod 4. In order to completely prevent generation of such scratches, the following countermeasure is taken. That is, external force is applied to the spring elements 6 to deflect the same such that the spring elements 6 are not in contact with the fuel rod 4. Subsequently, the fuel rod 4 is inserted into the grid cell. The external force is then relieved to urge the spring elements 6 against the fuel rod 4. When the external force is applied to the spring element 6 to deflect the same such that an amount of deflection thereof reaches a value equal to or above a predetermined value, however, the spring element 6 partially begins to reach its plastic range. The spring element 6 at the time the external force is relieved returns as indicated by the broken line in FIG. 8. Thus, the spring characteristics of the spring element 6 varies.
In view of the above, there has been provided a grid shown in FIGS. 9 and 10 in which, each of a plurality of spring elements 18 is formed such that, even if the spring element 18 is deflected as described above, the spring element 18 is deflected within an elastic range so that its spring characteristics do not vary.
The above grid is formed as follows. That is, each of a plurality of straps 11 has a wall whose pair of portions located respectively at both lateral sides of an elongated portion of the peripheral wall, which is to serve as the spring element 18, are punched to form a pair of punched sections 19. The elongated portion of the wall of the strap 11, which remains at the center between the pair of punched sections 19, bulges in the form of a triangle in cross-section, and a convex section 18a, which is to be held in pressure contact with the fuel rod 4, is formed at its apex portion.
The spring element 18 has the spring characteristics shown in FIG. 11. That is, the load increases linearly, accompanied with an increase in an amount of deflection and, further, the spring element 18 is deflected within an elastic range. Accordingly, the grid having the spring elements 18 constructed as above has such an advantage that, even if the spring element 18 is deflected so as not to be in contact with the fuel rod, the spring characteristics of the spring element 18 do not vary.
In the spring element 18, however, a range does not exist in which, even if the amount of deflection varies, the load is constant. Accordingly, because of variation during assembly of the grid, the amounts of deflection of the respective spring elements 18 are different from each other. Thus, it is difficult to clamp the fuel rod 4 with a constant load.
Moreover, when the spring element 18 is deflected, it tends to slide axially as illustrated in FIG. 12. For this reason, there is also such a disadvantage that it is difficult to maintain the height of the spring element 18 from the wall surface of the strap 11 under such a condition as assembling of the grid.